The present invention relates to a method and apparatus for detecting a fouled fluid filter, in particular such a method and apparatus used for aircraft engine fuel or oil feed circuits.
Fluids moving through the fuel or oil feed circuits of aircraft engines must be entirely free of impurities. It is known to filter these fluids using a fuel or oil filter which must be replaced when fouled or clogged with impurities. It is further known to install a bypass valve in parallel with the fluid filter wherein the valve opens when the fluid pressure drop across the filter between the filter inlet and filter outlet reaches a predetermined magnitude. The pressure drop will increase as the fluid filter becomes clogged with impurities. The bypass valve enables fluid to pass around the fluid filter so that the fluid will retain the desired pressure and that the circulation of the fluids throughout the aircraft engine will be unimpaired.
However, once the aircraft engine operates with an open bypass valve, the fuel or oil feed circuits may become contaminated. In order to monitor the status of the fluid filter, the filter pressure drop is permanently monitored by a detector, which typically comprises a pressure switch connected to an instrument or alarm in the cockpit of the aircraft. The drawbacks of such known systems include the fact that impurity clogging of the fluid filter will most likely arise when the aircraft engine is operating at maximum power and when the fluid passing through the filter is relatively cold.
In regards to an aircraft engine, the highest power output coupled with the coldest fluid temperatures will simultaneously occur during takeoff. The filter clogging alerts in most case arise during takeoff and in a significant number of cases, results in the aircraft stopping on the runway because the crew must stop the aircraft as soon as the filter clogging alarm is actuated. Generally, these aborted takeoffs are stopped from high speeds and may cause the aircraft to overshoot the end of the runway. Also, such aborted takeoffs degrade the airline's operational reliability and increase maintenance costs on brakes and tires of the aircraft.